Recovery of acetic acid from aqueous acetic acid solutions by extractive distillation



Sept. 8, 1953 G. A. R. HARTLEY 9 RECOVERY OF ACETIC ACID FROM AQUEOUS ACETIC ACID SOLUTIONS BY EXTRACTIVE DISTILLATION Filed NOV. 6, 1951 'ST: M

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. Faacnouarmc D\METHOXY PO YGl-(COL COLUMN 4 u 5 Boqdea //7ve/7/0r George AnfhonyRaf/zbone Harf/ey By his attorneys Patented Sept. 8, 1953 RECOVERY OF ACETI OUS ACETIC ACID TRACTIVE DISTILLA George A. R. Hartley, Coventry,

ACID FROM AQUE- SOLUTIONS BY EX- TION England, assignor to Courtaulds Limited, London, England, a

British company Application November 6 In Great Britain This invention relates to the recovery of acetic acid from aqueous acetic acid solutions by extractive distillation, and is a continuation-in-part application of my application, Serial No. 177,992, now abandoned.

In the specification of application, Serial No. 122,832, now abandoned, a process is described for the production of threads of cellulose acetate by extruding a solution of the cellulose acetate through a jet into an aqueous coagulant bath containing potassium acetate and also containing free acetic acid, preferably within the range of 50 to 250 grams of acetic acid per litre of bath. Cellulose acetate solutions employed in this process are preferably the acetic acid solutions obtained by the normal acetylation process after ripening and neutralisation of the acid catalyst have been eifected.

When the process described in the specification of the said application, Serial No. 122,832, is carried out continuously using an aqueous acetic acid solution of cellulose acetate together with either a potassium acetate or a potassium acetate-acetic acid coagulation bath, the concentra tion of acetic acid in the bath will increase under static conditions. It is therefore desirable to rectify the bath during continuous operation by withdrawin part of the bath liquor, either continuously or intermittently, recovering acetic acid from the withdrawn liquor and feeding the liquor, after any necessary further adjustment of the concentration of its constituents, to the spinning bath.

It is known to recover acetic acid from its aqueous solutions by fractional distillation, azeotropic distillation and extractive distillation. As the boiling points of water and acetic acid (100 centigrade and 118 centigrade respectively) are close together, a commercially practicable fractional distillation process requires a complex distillation column and a large expenditure of heat energy so that in general such processes are not employed commercially.

Azeotropic distillation, using azeotropic liquids such as ethyl acetate or amyl acetate, has been used commercially for the recovery of glacial acetic acid from aqueous acetic acid solutions such as are obtained for example in the manufacture of cellulose acetate. However the presence of potassium acetate in the acetic acid solution has been found to lead to difficulties, principally as follows:

(I) Water cannot be removed by extraction and rejection of the rafiinate since the latter will contain valuable potassium acetate. It is there- 1951, Serial No. 255,068

September 2, 1949 9 Claims. (01. 202-395) fore necessary to vaporise all the water and this results in a much greater heat requirement per unit weight of water removed.

(II) In the case of extraction of the spinning bath liquor potassium acetate reduces the partition coefiicient so that instead of complete extraction of the liquor being. obtained with two parts of ethyl acetate per part of aqueous feed liquor about six parts are required.

(III) The presence of potassium acetate in the spinning bath liquor increases the relative volatility of water with respect to acetic acid so that distillation gives a weaker acid than would be obtained if the salt were neutral or not present. Whereas a distillate containing 20 per cent acetic acid would be obtained in the absence of the salt, evaporation of the spinningbath liquor gives a 10 to 11 per cent acetic acid distillate.

It has also been proposed to separate acetic acid from its aqueous solutions by an extractive distillation process, for example in United States patent specification No. 2,350,256.

It is the object of the present invention to recover acetic acid from aqueous solutions of acetic acid particularly from the dilute aqueous acetic acid distillate obtained from solutions which also contain potassium acetate.

According to the present invention, a process for the recovery of acetic acid from aqueous solutions of acetic acid comprises adding a dimethoxy polyglycol chosen from the group consisting of (a) Dimethoxytetraethylene glycol (O(CH2.CH2.O.CH2.CH2.OCH3)2) (b) Dimethoxy triethylene glycol (CH3.O.CH2.CH2.OCH2.CH2.OCH2.CH2.OCH3) and (c) dimethoxy diethylene glycol (CH3.O-.CH2.CH2.O.CH2.CH2.O CH3) within the range of from 1 to 5:1.

The process according to the present invention employs the principle of extractive distillation in which an extraction solvent is added to the aqueous acetic acid solution to increase the volatility of the waterwith respect to the acid so that the water can be removed more readily and more completely than in the case of simple fractional distillation. Such an extraction solvent must be miscible in all proportions with and chemically inert to both water and acetic acid and must not form an azeotrope with either; it must also have a boiling point sufficiently higher than that of acetic acid to permit separation of the acid and the solvent by fractional distillation. Thus dimethoxy tetraethylene glycol boils at 276 centigrade, dimethoxy triethylene glycol at 220 centigrade and dimethoxy diethylene glycol at 161 centigrade. The dimethoxy polyglycols as defined are suitable extraction solvents which not only have the properties outlined above but also give a pronounced increase in the volatility of water with respect to the acid. As examples of the increase in volatility of water obtained by using dimethoxy tetraethylene glycol, in a mixture of 70 mols of dimethoxy tetraethylene glycol, 15 mols of acetic acid and 15 mols of water, the volatility of the water with respect to the acid is increased by a factor of 6 over the value obtained in the absence of the dimethoxy tetraethylene glycol; with a mixture of 50 mols oi dimethoxy tetraethylene glycol, 25 mols of acetic acid and 25 mols of Water, the relative volatility is increased -fold above the normal value for equimolecular mixtures of water and acetic acid. Similar increases in relative volatility are obtained with dimethoxy triethylene glycol and dimethoxy diethylene glycol.

The process according to the invention is par-s ticularly applicable to the recovery of acetic acid from dilute aqueous solutions of acetic acid containing for example up to about 15 per cent of acetic acid where the usual azeotropic dis? tillation processes are uneconomical. As described above, distillation of the potassium acetate spinning bath liquors used in carrying out the process of application, Serial No. 122,832 yields distillates containing to 11 per cent of acetic acid and such solutions are particularly suitable for rectification by the present invention.

The process for separating acetic acid and water using a dimethoxy glycol in accordance with this invention may be carried out in a continuously operated apparatus as illustrated in the accompanying drawing. The apparatus shown in the drawing comprises a distillation column I heated by a boiler 2 which may eonveniently be heated by steam under pressure. A mixture of acetic acid and water preferably in the vapour state, is fed into the column I at an intermediate point by way of a pipe 3 and the dimethoxy polyglycol is simultaneously fed in near the top of the column I by way of a pipe 4. The proportion of the dimethoxy polyglycol to the acid-water mixture is adjusted to give the required increase in the normal volatility of the water as indicated above. Heat is applied to the boiler 2, a suitable temperature for dimethoxy tetraethylene glycol being approximately 260 centigrade and steam driven off in the column 1 passes by way of av pipe 5 to a condenser E from which it may be withdrawn as desired by a waste pipe I. At the same time a mixture of acetic acid and the dimethoxy polyglycol collects in the boiler 2 and this mixture is passed by way of a pipe 9 to a second dis tillation column l0 heated by a boiler IL A suitable temperature for the boiler I I when using dimethoxy tetraethylene glycol is approximately 280 centigrade. In column 10 acetic acid is distilled ofi and the acid vapour passes by way of a pipe l2 to a condenser 13 from which the separated acetic acid may be withdrawn for reuse by a pipe I l. The dimethoxy polyglycol passes into the boiler H from the column 10 and thence into the feed pipe 4 so that it may be fed back to the column I for re-use. The presence of a small amount of acetic acid in the dimethoxy polyglycol which passes from the boiler H to the column I is not detrimental. An additional advantage of this continuously operated process is that the dimethoxy polyglycol is not distilled at any stage. In order to reduce the temperature in the boiler ll, the column I0 is preferably operated under reduced pressure.

For the rectification of spinning bath liquors containing potassium acetate as well as acetic acid, an evaporator is connected to the feed pipe 3 and the vapours recovered from the evaporator, which contain 10 to 11 per cent of acetic acid, are passed into the column l by way of pipe 3, as shown while potassium acetate is recovered from the evaporator.

When using a continuously operated apparatus of the type shown in the drawings with dimethoxy triethylene glycol the boiler 2 may conveniently be maintained at 150 centigrade with the boiler H at 220 centigrade and atmospheric pressure or at 168 centigrade and an absolute pressure of 150 mm. of mercury; with dimethoxy diethylene glycol corresponding tempera tures are centigrade for boiler 2 and 161 centigrade (at atmospheric pr ssure) for boiler ll.

What I claim is:

1. A process for the recovery of acetic acid from aqueous solutions of acetic acid which comprises adding a dimethoxy polyglycol chosen from the class consisting of dimethoxy tetraethylene glycoi, dimethoxy triethylene glycol and dimethoxy, diethylene glycol to the solution, fractionally distilling from the mixture substantially all the water present and then recovering acetic acid from the residual mixture of acetic acid and dimethoxy polyglycol.

2. A process for the recovery of acetic acid from aqueous solutions of acetic acid which comprises adding a dimethoxy polyglycol chosen from the class consisting of dimethoxy tetraethylene glycol, dimethoxy triethylene glycol and dimethoxy diethylene glycol to the solution, fractionally dis tilling from the mixture substantially all the water present and then iractionally distilling acetic acid from the residual mixture of acetic acid and dimethoxy polyglycol.

3. A process for the recovery of acetic acid from aqueous solutions of acetic acid which comprises adding a dimethoxy polyglycol chosen from the class consisting of dimethoxy tetraethylene glycol, dimethoxy triethylene glycol and dimethoxy diethylene glycol to the solution in an amount such that molecular ratio of the dimethoxy polyglycol to the water present in the solution is from 1 to 5:1, fractionally distilling from the mixture substantially all the water present and then fractionally distilling acetic acid from the residual mixture of acetic acid and dimethoxy polyglycol.

4. A continuous process for the recovery of acetic acid from aqueous solutions of acetic acid which comprises feeding the aqueous acid solution and a dimethoxy polyglycol chosen from the class consisting of dimethoxy tetraethylene gly- 01, dimethoxy triethylene glycol and dimethoxy diethylene glycol into a fractionating column, fractionally distilling substantially all the water from the mixture formed within the column, withdrawing the residual mixture of acetic acid and dimethoxy polyglycol from the fractionating column and passing it to a second fractionating column, fractionally the acetic acid-dimethoxy polyglycol mixture in the second fractionating column and returning the residual dimethoxy polyglycol to the first fractionating column.

5. A continuous process as claimed in claim 4 wherein the rates of feed of the aqueous acetic acid solution and of the dimethoxy polyglycol are so adjusted that the molecular ratio of the dimethoxy polyglycol to the water in the fractionating column is within the range of 1 to 5:1.

6. A continuous process as claimed in claim 4 wherein the second fractionating column is operated under reduced pressure.

7. A process for the recovery of acetic acid from aqueous solutions of acetic acid which comprises adding dimethoxy tetraethylene glycol to the solution, fractionally distilling from the mixture substantially all the water present and then recovering acetic acid from the residual mixture of acetic acid and dimethoxytetraethylene glycol. recovery of acetic acid from aqueous solutions of acetic acid which comprises adding dimethoxy triethylene glycol to the distilling ofl. acetic acid from solution, fractionally distilling from the mixture substantially all the water present and then recovering acetic acid from the residual mixture of acetic acid and dimethoxy triethylene glycol.-

9. A process for the recovery of acetic acid from aqueous solutions of acetic acid which comprises adding dimethoxy diethylene glycol to the solution, fractionally distilling from the mixture substantially all the Water present and then recovering acetic acid from the residual mixture of acetic acid and dimethoxy diethylene glycol.

G. A. R. HARTLEY.

References Cited in the file of this patent UNITED STATES PATENTS Number Name Date 1,624,811 Suida Apr. 12, 1927 2,123,348 Wentworth July 12, 1938 2,339,160 Dunn et al. Jan. 11, 1944 2,350,256 Shiras May 30, 1944 2,357,344 Norris et al. Sept. 5, 1944 2,430,086 Stafi Nov. 4, 1947 2,559,519 Smith et al. July 3, 1951 2,575,243 Carlson et al. Nov. 13, 1951 FOREIGN PATENTS Number Country Date 560,169 Great Britain Mar. 23, 1944 563,164 Great Britain Aug. 1, 1944 

1. A PROCESS FOR THE RECOVERY OF ACETIC ACID FROM AQUEOUS SOLUTIONS TO ACETIC ACID WHICH COMPRISES ADDING A DIMENTOXY POLYGLYCOL CHOSEN FROM THE CLASS CONSISTING OF DIMETHOXY TETRAETHYLENE GLYCOL, DIMETHOXY TRIETHYLENE GLYCOL AND DIMETHOXY, DIETHYLENE GLYCOL TO THE SOLUTION, FRACTIONALLY DISTILLING FROM THE MIXURE SUBSTANTIALLY ALL THE WATER PRESENT AND THEN RECOVERING ACETIC ACID FROM THE RESIDUAL MIXTURE OF ACETIC ACID AND DIMETHOXY POLYGLYCOL. 